Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilize high power base stations (NodeBs in 3GPP parlance) to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called Iub interface.
The second generation wireless communication system (2G), also known as GSM, is a well-established cellular, wireless communications technology whereby “base transceiver stations” (equivalent to the Node B′s of the 3G system) and “mobile stations” (user equipment) can transmit and receive voice and packet data. Several base transceiver stations are controlled by a Base Station Controller (BSC), equivalent to the RNC of 3G systems.
Communications systems and networks are developing towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network, and a System Architecture Evolution (SAE) solution, namely, an Evolved Packet Core (EPC), for a mobile core network. User Equipment (UE) can access a CS domain core network through a 2G/3GRAN such as the (Enhanced Data Rate for GSM Evolution, EDGE) Radio Access Network (GERAN) or a Universal Mobile Telecommunication System Terrestrial Radio Access Network (UTRAN), and access the EPC through the E-UTRAN.
Some User Equipments have the capability to communicate with networks of differing radio access technologies. For example, a user equipment may be capable of operating within a UTRAN and within an E-UTRAN.
Lower power (and therefore smaller coverage area) cells are a recent development within the field of wireless cellular communication systems. Such small cells are effectively communication coverage areas supported by low power base stations. The terms “picocell” and “femtocell” are often used to mean a cell with a small coverage area, with the term femtocell being more commonly used with reference to residential small cells. Small cells are often deployed with minimum RF (radio frequency) planning and those operating in consumers' homes are often installed in an ad hoc fashion. The low power base stations which support small cells are referred to as Access Points (AP's) with the term Home Node B (HNB's) or Evolved Node Node B (eHNB) identifying femtocell Access Points. Each small-cell is supported by a single Access Point. These small cells are intended to augment the wide area macro network and support communications to multiple User Equipment devices in a restricted, for example, indoor environment. An additional benefit of small cells is that they can offload traffic from the macro network, thereby freeing up valuable macro network resources. An HNB is an Access Point that provides a wireless interface for user equipment connectivity. It provides a radio access network connectivity to a user equipment (UE) using the so-called Iuh interface to a network Access Controller, also known as a Home Node B Gateway (HNB-GW). One Access Controller (AC) can provide network connectivity of several HNB's to a core network.
Typical applications for such Access Points include, by way of example, residential and commercial locations, communication ‘hotspots’, etc., whereby Access Points can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, small cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion or poor coverage at the macro-cell level may be problematic.
Thus, an AP is a scalable, multi-channel, two-way communication device that may be provided within, say, residential and commercial (e.g. office) locations, ‘hotspots’ etc, to extend or improve upon network coverage within those locations. Although there are no standard criteria for the functional components of an AP, an example of a typical AP for use within a 3GPP 3G system may comprise Node-B functionality and some aspects of Radio Network Controller (RNC) functionality as specified in 3GPP TS 25.467. These small cells are intended to be able to be deployed alongside the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment.
Herein, the term “small cell” means any cell having a small coverage area and includes “picocells” and “femtocells.”
In a planned cellular network, a so-called neighbor cell list is used to identify adjacent cells to each cell, to facilitate handover of UE communications from a “source” cell to a “target” cell when the strength or quality of the signal from the serving (source) cell, for example, becomes too poor to maintain the communication. The neighbor cell list is broadcast to UEs to enable a UE to receive and assess the suitability of continuing a communication by transferring the communication to an adjacent (neighbor) cell. A neighbor cell list contains, inter alia, frequency and scrambling code information for all of the cells whose coverage area overlaps with the cell, to allow the UE to be able to receive and decode transmissions from the neighboring cells and send measurement reports back to the RNC. An Access Point and a node B may be provisioned with information about neighbor cells which may include frequency, scrambling code and a cell ID.
Handovers between macro cells, between small cells and between a macrocell and a small cell and between cells operating with different radio access technologies are all possible.
If it is assumed that a user equipment (UE) is participating in an active call, the UE receives the neighbor cell list in a radio resource control (RRC) message from the RNC via its serving node B. The UE measures the broadcast transmissions from cells in the list in order to identify the best (generally closest) neighboring cells to consider as potential target cells if the signal from its serving (source) cell should deteriorate in strength or quality.
There may be instances where a UE needs to make measurements on a different frequency (ie. non-co-channel) or on a different radio access technology vis a vis the source cell in order to identify a handover candidate. This can happen if there are no co-channel cells available to handover to, in which case, the UE must be forced to look for other candidate cells on other frequencies (inter-frequency) or other Radio Access Technologies (Inter-RAT). In such instances, the RNC issues an instruction to the UE to enter the so-called “compressed mode.” In this mode, the RF synthesizers in the UE can be retuned temporarily to detect and measure non-co-channel frequencies. Typically, in compressed mode, transmission and reception are stopped for a short time and the measurements are performed on another frequency or RAT in that time, after which, transmission and reception resumes. If the UE has a second receiver then it can make measurements while still continuing with transmission and reception on the first receiver. Typically, a UE is not put into compressed mode until it reports a UE measurement indicating that the serving cell is getting weak (such as a UE measurement based on RSSI (received signal strength index), Ec/No ratio in the pilot channel (CPICH) or RSCP (received signal code power), for example. However, compressed mode has the disadvantage of using extra resources and may also adversely affect quality of service
The UE then monitors specified neighbor cells, identified in a RRC measurement control message, until one of them meets the specified criteria (eg. signal strength or quality). Once one of the neighbor cells meets the specified criteria, the UE sends a Measurement Report to the RNC via its serving node B. Based on the information in the Measurement Report, the RNC determines whether to perform handover for the UE.
As mentioned above, handovers between a macrocell and a small cell are possible. The coverage area of a small cell may be typically just a few meters in radius. Consider an example where a small cell is served by a 3G AP located in a retail premises and the coverage of the small cell extends outside the premises into the roadway. A user of a UE, initially in a call supported by a macrocell, walking past the retail premises at approximately 5 km/h would take about 8 seconds to cover a 10 m range. It typically may take the UE 1-2 seconds to trigger a handover into the small cell once the coverage from the small cell was deemed better than the macrocell coverage. Whilst the small cell is the dominant carrier the call will proceed well. However, as the user walks towards the extremity of the small cell there may be a very short period of time (1-2 seconds) before the call drops if it is not handed back to the macrocell. Setting up a UE measurement on a small cell can take typically 1-2 seconds. Furthermore, the reliability of these messages when the UE has just handed over from the macrocell to the small cell (when it is between two cells) is not guaranteed under poor RF conditions and in 3G systems there are typically 3 sets of NCell measurement types to configure (InterFreq, IntraFreq and InterRAT). Therefore, there may not be enough time for a handover back to the macrocell to be instigated and completed before the call is dropped from the small cell.